Large screen projection displays have been in demand for use in meetings, eduction, and for public announcement. Such systems are typically used in brightly lighted locations such as offices, airport lobbies and other public places. Thus, a bright screen with multicolored images is very important.
To satisfy the demand for such display systems, several types of liquid crystal display panel systems have been proposed. Such systems, however, have typically been limited in the number of different colors produced, or have exhibited poor contrast ratios. Such limitations result from various factors. For example, the physical and electrical characteristics of the individual liquid crystal display panel assemblies for producing colors, typically vary in actual production.
In a stacked display panel construction, a series of display panels and associated polarizers or filters are arranged along an optical path. The relative luminance of the individual panels is also effected by the order in which the panel assemblies are disposed within a stacked arrangement; i.e., a panel disposed more closely to the light source generally exhibits greater light transmittance characteristics as compared to a panel which is disposed more remotely from the light source along the common optical path because of the induced heating by the light source.
Another problem with conventional liquid crystal display panel systems is that the light transmission characteristics of the individual panels in a stacked panel arrangement, also vary in actual production. Consequently, the contrast levels of a multiple number of panels arranged along an optical path, does not follow in a linear manner, thereby resulting in color distortion.
Not only do the abstract values differ from panel to panel in a stacked configuration where each panel is responsible for a portion of the visible color spectrum, but also the excitation or gamma curves (applied voltage versus relative luminance) for each panel vary widely in significant manners. Accordingly, even if the panels could somehow be matched at one intensity or shading level, it would be difficult, if not impossible, to cause the contrast levels of the stacked panel arrangement to be balanced from one panel to the next, in a stacked panel arrangement.
Because typical liquid crystal display panel exhibits a hysteresis effect as a function of the applied voltage, different hues or shades of color can be produced by multiplexing the individual pixels or by applying incremented voltage steps between the excited and unexcited states of the panel. While such a technique may produce colors with multiple hues, it has been difficult, if not impossible, for such panel systems to produce a large number of different colors with multiple shading levels, because the relative luminance between the different colored panel assemblies varies to such a great extent.
For example, in U.S. Pat. No. 4,416,514, there is disclosed a liquid crystal color filter, which includes a set of differently colored dichroic polarizers interposed with an equal number of voltage responsive twisted nematic liquid crystal cells, and a neutral polarizer. Each of the above described elements are arranged along an optical path in a predetermined manner for modifying the spectral content of visible light incident to the filter to produce any one of eight predetermined colors. Shades of the predetermined colors are achieved by varying the voltage applied to the individual liquid crystal cells.
While the above described patented system may produce a full colored display image, it was limited to only an eight color system, since it would be difficult, if not impossible, to balance the filter colors for different hues on a pixel by pixel basis, as the light transmission characteristics of each of the liquid crystal display panels vary slightly, due to manufacturing inconsistencies between like panels and due to the physical and electrical characteristics of the panels. Thus, color balancing for the entire unit is generally unsatisfactory for some applications. More particularly, if the generating voltage levels are varied by same amount for each panel, each panel will have a different amount of relative luminance, thereby resulting in a distortion of the resulting colored image on a pixel by pixel image basis.